This patch is required CRIU project (www.criu.org).
To migrate processes with posix timers we have to make sure, that we can
restore posix timer with proper id.
Currently, this is not true, because timer ids are allocated globally.
So, this is precursor patch and it's purpose is make posix timer id to be
allocated per process.

Patch replaces global idr with global hash table for posix timers and
makes timer ids unique not globally, but per process. Next free timer id is
type of integer and stored on signal struct (posix_timer_id). If free timer id
reaches negative value on timer creation, it will be dropped to zero and
-EAGAIN will be returned to user.

Note: with this patch, id, returned to user, is not the minimal free
amymore. It means, that id, returned to user space in loop, listed below, will
be increasing on each iteration till INT_MAX and then dropped to zero:

v6:
1) include of "linux/idr.h" removed from "include/linux/sched.h"
2) include of "linux/idr.h" removed from "kernel/posix-timers.c"
3) per-process id overlap on timer add fixed

v5:
1) Patch changelog updated

v4:
1) a couple of coding style fixes (lines over 80 characters)

v3:
1) hash calculation simlified to improve perfomance.

v2:
1) Hash table become RCU-friendly. Hash table search now done under RCU lock
protection.
I've tested scalability on KVM with 4 CPU. The testing environment was build
of 10 processes, each had 512 posix timers running (SIGSEV_NONE) and was
calling timer_gettime() in loop. With all this stuff being running, I was
measuring time of calling of syscall timer_gettime() 10000 times.

/*
- * Management arrays for POSIX timers. Timers are kept in slab memory
- * Timer ids are allocated by an external routine that keeps track of the
- * id and the timer. The external interface is:
- *
- * void *idr_find(struct idr *idp, int id); to find timer_id <id>
- * int idr_get_new(struct idr *idp, void *ptr); to get a new id and
- * related it to <ptr>
- * void idr_remove(struct idr *idp, int id); to release <id>
- * void idr_init(struct idr *idp); to initialize <idp>
- * which we supply.
- * The idr_get_new *may* call slab for more memory so it must not be
- * called under a spin lock. Likewise idr_remore may release memory
- * (but it may be ok to do this under a lock...).
- * idr_find is just a memory look up and is quite fast. A -1 return
- * indicates that the requested id does not exist.
+ * Management arrays for POSIX timers. Timers are now kept in static hash table
+ * with 512 entries.
+ * Timer ids are allocated by local routine, which selects proper hash head by
+ * key, constructed from current->signal address and per signal struct counter.
+ * This keeps timer ids unique per process, but now they can intersect between
+ * processes.
*/